This invention relates to adhesive dispensing systems, and, more particularly, to an adhesive dispensing apparatus for applying multiple, parallel uniform beads of adhesive continuously onto one portion of a substrate and intermittently onto another portion of a substrate.
One product which acquires the application of multiple, parallel, uniform beads of adhesive is disposable diapers. In the manufacture of disposable diapers, multiple, parallel, uniform beads of pressure-sensitive adhesive are applied to a moisture impervious backing sheet of the diaper so as to adhere the backing sheet to the absorbent pad of the diaper. To ensure secure attachment of these layers, by means of an economical quantity of adhesive while obtaining an acceptable visual appearance of the resulting product, the adhesive beads must be accurately positioned along the backing sheet and formed in fine, uniform width beads.
It has been the practice in prior art methods of making disposable diapers to employ a metering gear head positioned above a moving layer of the plastic backing sheet to apply multiple, parallel beads of pressure-sensitive adhesive to the plastic backing sheet for subsequent attachment to an absorbent pad. Metering gear heads include a plurality of spaced discharge orifices which are each supplied with adhesive from a separate gear pump for applying multiple, parallel beads of adhesive on the plastic backing sheet. Although metering gear heads apply adhesive beads on a substrate with good accuracy, and dispense beads of uniform size and width, there are several problems in the use of metering gear heads for the manufacture of disposable diapers.
One problem with metering gear heads is that they are relatively heavy and bulky, making it difficult to mount them in close proximity on a diaper manufacturing line. The size of metering gear heads is attributable, in part, to the fact that each bead they dispense on a surface requires a separate gear pump and an associated drive motor to control the flow of adhesive forming the bead. The use of separate gear pumps for dispensing each bead contributes to high cost of the metering equipment, and results in a relatively complex metering device. As a result, the cost for maintaining the equipment is very appreciable.
It is desirable in some applications to apply a plurality of continuous, parallel beads onto one portion of a surface and spaced or interrupted beads on another portion of the surface. In the manufacture of disposable diapers, cut-outs are made at intervals in the diaper material for the leg holes. A substantial savings can be realized if adhesive is applied intermittently to the outer edges of the diaper, to leave a gap without adhesive where the leg holes are cut, while continuously applying adhesive to the center portion of the diaper.
Unfortunately conventional metering gear heads have not been successfully used to apply intermittent, uniform beads of adhesive upon a substrate such as the plastic backing sheet of a diaper. If metering gear heads are operated intermittently to dispense adhesive, they produce a substantial cut-off drool when turned off, and then do not immediately provide consistent flow when turned back on. A disposable diaper manufactured by an intermittently operated metering gear head would therefore have uneven and/or varying width adhesive beads on both sides of the backing sheet where the leg hole is cut, which is visually unacceptable. Metering gear heads are thus operated continuously in the manufacture of disposable diapers, applying continuous multiple beads across the entire width of the backing sheet of the diaper, which results in a substantial waste of adhesive where the leg holes are cut in the backing sheet.